1. Field of the Invention
The present invention relates generally to an image forming apparatus and, more particularly, to an electrophotographic image forming apparatus, such as a color laser printer or a color digital copier.
2. Description of the Related Art
In an image forming apparatus, such as an electrophotographic color digital copier or color laser printer, the image density of the printed material or copied material is changed in some cases depending on a change in the environmental conditions or lapse of time. In a multi-gradation type printer or digital copier, as well as an analog copier, it is important to suppress changes in image density so as to stabilize the image density of the printed or copied material.
In a conventional image forming apparatus, some tolerance is imparted to the material forming the apparatus itself and to the image forming process itself such that the image density of the printed material or copied material can be stabilized by a maintenance operation. With the conventional technique, however, the tolerance imparted to the material and the image forming process itself has its own limit. Also, maintenance is laborious and costly. In addition, the period of change in the image density of the printed material or copied material is shorter than the frequency of maintenance. It follows that it is impossible to obtain a stable image density of the printed material or copied material by maintenance alone.
Recently, a new technique is being proposed for correcting the change in the image density, which is caused by the change in the environmental condition or by the lapse of time, in an interval shorter than that of normal maintenance operations. In the new technique, the amount of developing agent (toner) attached to an image carrier (photosensitive drum) included in an image forming apparatus is measured by a toner measuring means using an optical element such as a photo sensor. The measured value is compared with a reference value covering the case where a toner is not attached to the image carrier. Based on the result of the comparison, the image forming conditions, such as the charging amount of the image carrier, developing bias voltage, light-exposure amount, and toner concentration, are changed to keep constant the amount of the toner attached to the image carrier. The image forming apparatus employing the particular technique makes it possible to obtain a stable image density all the time.
However, the apparatus employing the new technique described above leaves room for further improvement. In the new technique, it is necessary to measure the amount of light reflected from the image carrier itself. What should be noted is that the light reflection from the image carrier is not uniform, but changes periodically. Where measuring of the toner amount attached to the image carrier is affected by the nonuniform light reflection from the image carrier, it is certainly possible to offset the range of change caused by the nonuniform light reflection by the comparison noted above between the measured value and the reference value. However, the measuring of the toner amount attached to the image carrier is scarcely affected, in general, by the nonuniform light reflection from the image carrier in the case where the toner attached to the image carrier. It follows that the nonuniform light reflection from the image carrier itself is output as it is, leading to errors of measurement and making it impossible to measure accurately the toner amount attached to the image carrier.
What should also be noted is that, where an image forming apparatus is enabled to perform a certain number of gradations, the values measured by the toner amount measuring means tend to have a narrow detection range in the high density stage and a broad detection range in the low density stage. As a result, the accuracy of measurement is impaired, if a random reading appears in the stage of transferring the measured result.
An analog/digital (A/D) converter of high resolution is required for detecting an output signal having a narrow detection range in the high density stage and a broad detection range in the low density stage. However, a A/D converter of high resolution is costly, in general, leading to increased manufacturing costs for the entire image forming apparatus.
As described above, the conventional image forming apparatus is defective in that, in measuring the toner amount attached to the image carrier by using a toner amount measuring means, errors of measurement are caused by a nonuniform light reflection from the image carrier itself. Also, the accuracy of measurement is impaired by the mixing of a noise in the step of transferring a signal denoting the result of measurement. Further, there is a difference in the range of detection in respect of the light reflectivity of the toner. Under the circumstances, the conventional image forming apparatus is incapable of maintaining a stable image density.